FROM THE FARM: ‘Dry rot’: No such thing in wood

Adam Taylor, professor of wood products with University of Tennessee Forestry Extension, shares some good information with us this week:

Last summer there was a tragic failure of a wood-framed balcony on an apartment building in Berkeley, Calif. Six young people were killed when the fourth-floor balcony they were standing on suddenly fell to the ground. Speculation immediately after the accident, which subsequent investigations confirmed, was that the wood supports for the balcony were rotten, as a result of the wood being exposed to water during and after construction.

However, in news reports there is repeated mention of ‘dry rot’. This is a misnomer that may be misleading. Water is the basis of life. This is true for animals and plants but it is also true for the fungi. The kingdom Fungi includes organisms such as the mushrooms we buy in the grocery store as well as the wood-rotters that can cause significant damage to wood in buildings. There are many species of decay fungi and they are grouped by the appearance of the wood after attack, for example: brown rot and white rot. Regardless of the type of wood decay, all require that the wood be wet.

Dry rot is a commonly-heard term. Usually when wood decay is identified as ‘dry rot’ it is a misidentification of brown rot. In the advance stages of brown rot, all that remains of the wood is a spongy brown material that can easily be broken by hand. If this dries out after the fungus has run its course, the dusty, crumbly residue will then be both dry and rotten, but it is still correctly referred to as brown rot. Wood decay caused by the brown rot fungus Serpula lacrymans (true dry rot)

Most decay fungi feed on wood that is exposed to a steady source of water. This can occur if wood is in contact with the ground, or if a leak in a building traps water in contact with the wood. There are a few, relatively rare, fungi that can transport the needed water over a short distance to the wood. These species have been called the ‘dry rot fungi’ but they are actually water-transporting brown rotters. Meruliporia incrassata or “Poria” is one such fungus that is found in the southern United States.

Poria is very susceptible to drying, so it is very unlikely to become established on wood that is in a dry environment. Thus, even though Poria can move water to wood that would otherwise be too dry to support decay, prevention of this fungus is similar to that for other wood rotters: Keep the wood dry and, for extra protection where needed, use preservative chemical treatments.

Tragedies like the balcony collapse in San Francisco are rare. However, damage to wood caused by water exposure where the wood is expected to remain dry is all too common. To ensure durable, safe, wood-framed structures, a ‘dry design’ should be supplemented with strategies to cope with unexpected wetting.